Field of the Invention
The present invention relates to wireless communication and, more particularly, to a device to device (D2D) communication method performed by user equipment in a wireless communication system and user equipment supporting the same.
Related Art
In International Telecommunication Union Radio communication sector (ITU-R), a standardization task for International Mobile Telecommunication (IMT)-Advanced, that is, the next-generation mobile communication system since the third generation, is in progress. IMT-Advanced sets its goal to support Internet Protocol (IP)-based multimedia services at a data transfer rate of 1 Gbps in the stop and slow-speed moving state and of 100 Mbps in the fast-speed moving state.
For example, 3rd Generation Partnership Project (3GPP) is a system standard to satisfy the requirements of IMT-Advanced and is preparing for LTE-Advanced improved from Long Term Evolution (LTE) based on Orthogonal Frequency Division Multiple Access (OFDMA)/Single Carrier-Frequency Division Multiple Access (SC-FDMA) transmission schemes. LTE-Advanced is one of strong candidates for IMT-Advanced.
There is a growing interest in a Device-to-Device (D22) technology in which devices perform direct communication. In particular, D2D has been in the spotlight as a communication technology for a public safety network. A commercial communication network is rapidly changing to LTE, but the current public safety network is basically based on the 2G technology in terms of a collision problem with existing communication standards and a cost. Such a technology gap and a need for improved services are leading to efforts to improve the public safety network.
The public safety network has higher service requirements (reliability and security) than the commercial communication network. In particular, if coverage of cellular communication is not affected or available, the public safety network also requires direct communication between devices, that is, D2D communication.
D2D communication may have various advantages in that it is communication between devices in proximity. For example, D2D UE has a high transfer rate and a low delay and may perform data communication. Furthermore, in D2D communication, traffic concentrated on a base station can be distributed. If D2D UE plays the role of a relay, it may also play the role of extending coverage of a base station.
Meanwhile, one of important procedures when D2D communication is performed is to discover another device in proximity. To this end, D2D UE sends a discovery request signal (it may be hereinafter called a discovery request signal), and another D2D UE may send a discovery response signal in response to the discovery request signal.
Furthermore, in D2D communication, direct communication may be performed between two types of UE.
Meanwhile, when RRC connection is established between the Radio Resource Control (RRC) layer of UE and the RRC layer of a base station, the UE is in the RRC_CONNECTED state. If RRC connection is not established, the UE is in the RRC_IDLE state.
UE in the RRC_IDLE state may have various substates depending on the type of cell from which service is provided. In some substates, it may be not appropriate to perform D2D communication because UE does not perform time alignment or does not sufficiently secure reliable information.
Accordingly, there is a need for a D2D communication method in which various substates of UE in the RRC_IDLE state are taken into consideration and UE supporting the method.